This invention relates to the field of ocular refractive procedures, specifically to a method of changing the curvature of the cornea of the eye.
The condition of ametropia, in which images are not focused properly on the retina due to some refractive error in the eye, is pervasive in the general human population. Depending on the specific population considered, from 25% to 50% of persons have some degree of ametropia. The most common treatment for this condition is to place a corrective lens in the optical path, e.g. spectacles or contact lenses. Since conventional corrective measures have serious drawbacks, such as inconvenience and discomfort, there is substantial activity in the field of ophthalmology devoted to correcting ametropia by altering the optics of the eye itself. Most of this activity is concentrated on altering the cornea, which is the strong transparent membrane covering the central anterior portion of the eye.
The cornea contributes the greatest amount of refraction to the overall optical system of the eye due to its highly curved surface. Accordingly, small changes in corneal curvature can produce substantial changes in the focus of the eye. The cornea has five structural layers: epithelium, Bowman""s membrane, stroma, Descemet""s membrane, endothelium. The main load-bearing structures in the cornea are the uniform-diameter (about 30 nm) collagen fibrils within the many (several hundred) lamellae of the stroma. Much of the activity thus far has focused on altering the structure of the stroma within the cornea.
Several surgical methods have been used to alter the refractive properties of the cornea. A first surgical approach, Radial Keratotomy (RK), uses radial incisions to weaken the paracentral cornea in the circumferential direction by severing some collagen fibrils. Thus, this procedure can lead to circumferential expansion and consequent flattening of the optical zone in the central cornea. Another surgical approach removes layers of the stroma in specific patterns to achieve a xe2x80x9csculptingxe2x80x9d of the corneal surface. Such removal can be by a keratome (knife) as in Automated Lamellar Keratoplasty (ALK), or by a laser, as in Photorefractive Keratectomy (PRK) and Laser in-situ Keratomileusis (LASIK). In PRK, material is ablated by laser directly from the anterior surface, thus destroying Bowman""s layer and associated nerve endings. In LASIK, a flap of the cornea is first cut and folded back, so that only stromal layers are ablated. PRK, ALK, and LASIK all result in a thinned cornea. Thus, the remaining layers of the cornea must support the intraocular pressure. As a result, the optical effect of these surgeries depends in part on the stress-strain behavior of the remaining material.
Each of the surgical methods has serious side effects. The wounds from RK take several months to heal, and the resultant scarring tends to scatter light, leading to the perception of a xe2x80x9cstarburstxe2x80x9d effect. RK, PRK, ALK, LASIK, and their variants are invasive or destructive, require time for wound healing, alter the natural structure of the cornea, offer limited predictability and stability, and often require similarly invasive corrections to maintain or improve the corrective alterations.
A non-surgical method of altering the shape of the cornea is through thermokeratoplasty, which heats selected locations of the cornea to at least the xe2x80x9ccollagen shrinkage temperaturexe2x80x9d, causing the collagen fibrils to contract to about one-third their original length. Initially, heated wires were applied to the cornea, but it was found that the resulting collagen shrinkage was not stable, possibly due to the healing response of the tissue. Other methods of thermokeratoplasty have been proposed, such as using microwave radiation, radiofrequency antennae, ultrasound transducers, and lasers. The wide use of thermokeratoplasty was hindered, however, by the high temperature necessary for shrinkage of the collagen fibrils. Such high temperatures may permanently damage the more sensitive components of the cornea, especially the endothelium.
There remains a need for a safe, effective, non-invasive method of permanently altering the shape of the cornea to correct ametropia. There is also a need for an apparatus designed to practice this method of altering corneal shape.
A method and apparatus for non-invasively altering corneal curvature to correct ametropia by the combined application of heat and pressure differential is disclosed. The invention features an apparatus with a means for heating a selected portion of the cornea, and a means for the application of a pressure differential (e.g. suction) to a selected portion of the cornea. The apparatus uses the simultaneous combination of heat and suction to permanently stretch some of the collagen fibrils within the cornea, resulting in a change in the shape of the cornea and thus the focus of the eye. The invention also features a method for simultaneously combining heat and suction to cause a permanent change in the shape of the cornea by causing a stretching of the collagen fibrils within the eye.
The apparatus of the present invention may apply the heat and the suction simultaneously to the same area of the eye. Alternatively, heat may be applied to one area while a pressure differential is applied to another, possibly overlapping, area or heat may be applied followed by suction within a sufficiently short period of time that the eye remains heated to a point where it can be permanently deformed with pressure. The application of both heat and suction may be done cyclically, and may vary in the ratio of heating cycles to suction cycles. For example, each heat cycle may correspond to a single suction cycle. Alternatively, multiple suction cycles may take place within a single heating cycle. The amount of time and the temperature at which heat is applied can be varied. Further, the amount of time and magnitude at which a pressure differential is applied can be varied. Each is varied and applied so as to (1) substantially eliminate any damage to the eye and (2) permanently change the shape of a component of the eye to correct the point at which the eye focuses light thereby resulting in improved vision for the patient. To accomplish such the apparatus may remain in contact with the eye between therapeutic applications, or may be removed between applications.
A preferred embodiment of the invention uses a moving heated fluid to raise the temperature of a selected area of the cornea. Preferably, the fluid is an aqueous saline solution having a pH and ion concentration in osmotic balance with the cornea. The temperature of the fluid must be sufficient to allow the collagen fibrils to creep at a reasonable rate, but must not be sufficiently high as to result in collagen shrinkage. Preferably, the temperature is between 45xc2x0 C. and 55xc2x0 C. for a human eye. An alternative embodiment of the invention uses an infrared radiation source to raise the temperature of a selected area of the cornea. In a preferred embodiment, the heating is applied for a period of 1-3 minutes in each heating cycle.
The invention features a vacuum manifold for the application of a pressure differential to the eye. In a preferred embodiment, the magnitude of suction applied is between 100 mmHg and 500 mmHg. Suction may be applied for a period of between 1 second and 3 minutes. In a more preferred embodiment, the suction is applied for a period of 15-30 seconds in each suction cycle.
A primary object of the invention is to provide a method of altering the shape of one or more components of an eye which is preferably a human eye by heating a predetermined heating zone of the eye to a therapeutically effective temperature which is below the collagen shrinkage temperature of the cornea and applying a therapeutically effective amount of pressure (e.g., vacuum) to the eye in order to stretch a component of the eye such as the cornea, a predetermined amount thereby permanently altering the shape of the eye in a manner so as to improve the focus of the eye and the patients resulting vision.
Another object of the invention is to provide such a method wherein the heating is carried out by the flow of a heated fluid such as an aqueous saline solutions and/or by the application of other heating means such as electromagnetic radiation which may be infrared radiation in amounts so as to raise the temperature of the heating zone to a temperature in the range of about 45xc2x0 C. to about 55xc2x0 C. for a period of time in the range of about 1-5 minutes.
An advantage of the invention is that the shape of the eye can be changed so as to correct vision without cutting any of the components of the eye.
Another object of the invention is to provide a device for changing the shape of the eye and particularly for changing the shape of the cornea of the eye, which device is comprised of a manifold having an outwardly extending surface which conforms to the outer surface of the eye in a manner so as to create a seal between the eye and the extending surface, which manifold includes an open channel leading to a first opening in the manifold wherein the channel is connectable to a vacuum source and further wherein the manifold is preferably connectable to a heating source which may be an electromagnetic heating source or a fluid pumping source.
It is an object of the present invention to provide a method for changing the curvature of the eye or component thereof such as the cornea sufficiently to correct or reduce refractive errors of the eye by a non-invasive and nondestructive procedure, leaving the eye intact and undamaged.
It is another object of the invention to provide an apparatus for the non-invasive, nondestructive therapeutic application of heat and suction to correct or reduce refractive errors of the eye.
An advantage of the invention is improved post-treatment stability, strength, and clarity.
A feature of the invention is that it may be practiced on an eye that has undergone pretreatment.
These and other objects, advantages and features of the present invention will become apparent to those skilled in the art upon reading the disclosure.